The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.
(1) Field of the Invention
The present invention relates to the calibration of preamplifiers and more particularly, to a calibration circuit for use with a differential input, monolithic integrated circuit preamplifier.
(2) Description of the Prior Art
A preamplifier is commonly used with a sensor, such as a hydrophone, to amplify low level signals received from the sensor. Such preamplifiers can be constructed in a single monolithic integrated circuit in small sizes. The small size of such preamplifiers allows placement close to the sensor without adversely affecting the sensor""s performance. One such preamplifier is disclosed in U.S. Pat. No. 5,339,285, incorporated herein by reference.
A calibration circuit can be used with the sensor and preamplifier to test the integrity of the sensor and preamplifier. One method for calibrating a differential input amplifier involves injecting a single ended signal at a point past the input stage of the amplifier. One problem with this method is that the input stage, the sensor, and the interconnect wiring are not tested. According to another method, a differential signal is injected at the inputs to the amplifier. Although this method is capable of testing all circuitry, this method has required complicated wiring schemes and switching to prevent both unwanted noise coupling via the calibration circuitry and an impedance imbalance that would degrade the amplifier""s common mode rejection.
Another method of calibration is disclosed in U.S. Pat. No. 4,648,078. This patent, however, discloses calibration of a single ended preamplifier, not a differential preamplifier.
Accordingly, one object of the present invention is to calibrate a preamplifier as well as its associated sensor and interconnect wiring without requiring overly complicated wiring schemes and switching.
Another object of the present invention is to provide calibration circuitry for a differential input, monolithic integrated circuit preamplifier, which can easily be implemented on a single integrated circuit.
The present invention features a calibration circuit for a differential input preamplifier having first and second preamplifier inputs. The calibration circuit comprises first and second capacitors connected to respective first and second preamplifier inputs. The first and second capacitors have different capacitance values. A calibration signal source is connected between the first and second capacitors for applying a calibration signal.
The present invention also features a calibrated preamplifier for use with a sensor. The calibrated preamplifier comprises a differential input, single-ended output amplifier and a calibration circuit connected across first and second inputs of the amplifier. The calibration circuit applies a calibration signal to create a difference voltage between the inputs.
In one preferred embodiment, the amplifier and the calibration circuit are implemented on a monolithic integrated circuit. In one embodiment, only the calibration capacitors are implemented on the monolithic integrated circuit. In another embodiment, the calibration capacitors and the calibration signal source are both implemented on the monolithic integrated circuit.
The present invention also features a sensor system comprising a sensor having first and second output leads and a differential input, single ended output amplifier having first and second input leads connected to the first and second output leads of the sensor. A calibration circuit is connected across the first and second output leads of the sensor and the first and second input leads of the differential input single ended output amplifier. The calibration circuit applies a calibration signal to create a difference voltage between the amplifier input leads such that the preamplifier output voltage indicates the presence or absence of sensor impedance.
The preamplifier preferably includes first and second input resistors connected across the first and second input leads to form an analog filter with the sensor capacitance. The capacitance values of the first and second capacitors are preferably less than the capacitance of the sensor. In one embodiment, the sensor is a hydrophone. In another embodiment, the sensor is an accelerometer.